74 W. H. F. ADDISON AND J. L. APPLETON, JR. 
layer of the enamel-organ becomes sinuous, and low papillae 
are being formed. 
On the lingual side, the structure of the basal end of the enamel- 
organ is similar, but somewhat simpler. Thus there are three 
layers at and near the basal margin, but soon, proceeding an- 
teriorly, these become reduced to two by the disappearance of 
the middle layer. The lingual side then consists of two rows 
of cuboidal or flattened cells, one constituting the outer and the 
other the inner layer of the enamel-organ in this situation. 
The dental papilla is made up of closely packed small stellate 
cells, with rounded nuclei. The mesenchymal cells which lie 
against the basal margin of the enamel-organ are rounded or 
irregular in shape, but within a short distance (0.5 mm.) anterior 
to this margin, the peripheral cells become first cubical and then 
columnar in shape. Where they are beginning to form dentine 
they measure 30 in length. From the odontoblasts processes 
enter the dentinal tubules of the dentine. The outer surfaces 
of the odontoblasts from which these processes arise show a 
distinct cuticular margin. Between the odontoblasts at short 
intervals capillaries form loops around the cells. These are 
evidently for the purpose of insuring an ample blood supply 
to these functionally active cells. 
Four days old 
By 4 days of age there has been continued growth, and deposi- 
tion of enamel and dentine. The upper incisor measures 3.6 
~ 
mm. in length and the lower 5 mm. The position of the apex 
of the tooth is in close relation to the oral epithelium (fig. 18). 
A thickened mass of epithelium, partly a derivative of the origi- 
nal stalk of the enamel-organ, and partly an ingrowth from the 
Fig. 18 Longitudinal section of upper incisor of 4-day albino rat, showing 
the increased curvature of the outline of the tooth and the relation of the apex 
of the tooth to the ingrowth of the oral epithelium. X 16. 
Fig. 19 Longitudinal section through basal end of labial side of enamel- 
organ of 4-day albino rat showing the region of the thickened margin. a, mar- 
gin composed of mass of proliferating cells; b, region where three layers are seen; 
c, region where stratum intermedium becomes differentiated from rest of middle 
layer. Anterior to the region of this figure the epithelial papillae appear and 
the ameloblasts begin to form enamel. X 80. 
